Olivine

A group of rock-forming minerals found in Earth's crust.
An abundant mineral in Earth's mantle. A
constituent of many meteorites.

What is Olivine?

Olivine is the name of a group of rock-forming minerals
that are typically found in mafic and ultramafic igneous
rocks such as basalt,
gabbro,
dunite, diabase, and peridotite.
They are usually green in color and have
compositions that typically range between Mg2SiO4
and Fe2SiO4. Many people are
familiar with olivine because it is the mineral of a
very popular green gemstone known as peridot.

Geological Occurrence of Olivine

Most olivine found at Earth's surface is in
dark-colored igneous rocks. It usually crystallizes
in the presence of plagioclase
and pyroxene to form gabbro or basalt. These types
of rocks are most common at divergent plate
boundaries and at hot spots within the centers of
tectonic plates.

Olivine has a very high crystallization temperature
compared to other minerals. That makes it one of the
first minerals to crystallize from a magma.
During the slow cooling of a magma, crystals of
olivine may form and then settle to the bottom of
the magma chamber because of their relatively high
density. This concentrated accumulation of olivine
can result in the formation of olivine-rich rocks
such as dunite in the lower parts of a magma
chamber.

Crystals of olivine are sometimes formed during the
metamorphism
of a dolomitic
limestone or dolomite.
The dolomite contributes magnesium, and silica is
obtained from quartz
and other impurities in the limestone. When olivine
is metamorphosed, it is transformed into serpentine.

Olivine is one of the first minerals to be altered
by weathering. Because it is so easily altered by
weathering, olivine is not a common mineral in sedimentary
rocks and is only an abundant constituent of
sand
or sediment when the deposit is very close to the
source.

Composition of Olivine

Olivine is the name given to a group of silicate minerals
that have a generalized chemical composition of A2SiO4.
In that generalized composition, "A" is usually Mg
or Fe, but in unusual situations can be Ca, Mn, or
Ni.

The chemical composition of most olivine falls
somewhere between pure forsterite (Mg2SiO4)
and pure fayalite (Fe2SiO4).
In that series, Mg and Fe can substitute freely for
one another
in the mineral's atomic structure - in any ratio.
This type of continuous compositional variation is
known as a "solid solution" and is represented in a
chemical formula as (Mg,Fe)2SiO4.

The name "olivine" is used instead of "forsterite"
or "fayalite" because a chemical analysis or other
detailed
testing is needed to determine which
one is dominant - if either is dominant. The name
"olivine" serves as a quick, convenient, and
inexpensive way to put a name on the material. A
list of the more common olivine minerals and their
composition is given in the table below.

Olivine Minerals

Mineral

Chemical
Composition

Forsterite

Mg2SiO4

Fayalite

Fe2SiO4

Monticellite

CaMgSiO4

Kirschsteinite

CaFeSiO4

Tephroite

Mn2SiO4

Olivine receives its name from its usual olive-green
color. Many geology students remember the color
of olivine by using a rhyme: "olivine is green."
That rhyme is true with most classroom specimens;
however, there are rare iron-rich olivines
(fayalites) that are brownish in color.

Olivine in Earth's Mantle

Olivine is thought to be an important mineral in
Earth's mantle. Its presence as a mantle mineral has
been inferred by a change in the behavior of seismic
waves as they cross the Moho
- the boundary between Earth's crust and mantle.

The presence of olivine in Earth's interior is also
confirmed by the presence of olivine in xenoliths,
which are thought to be pieces of the upper mantle
delivered to Earth's surface in the magmas of
deep-source volcanic
eruptions. Olivine is also an abundant mineral
in the lower portion of many ophiolites. These are slabs of oceanic crust (with
part of the upper mantle attached) that have been thrust up onto an island or a continent.

Physical Properties of Olivine

Olivine is usually green in color but can also be
yellow-green, greenish yellow, or brown. It is
transparent to translucent with a glassy luster and
a hardness
between 6.5 and 7.0. It is the only common igneous
mineral with these properties. The properties of olivine are summarized in the table below.

Physical Properties of Olivine

Chemical Classification

silicate

Color

usually olive green, but can be yellow-green to bright green, iron-rich specimens are brownish-green to brown

Extraterrestrial Olivine

Olivine has been identified in a large number of stony
and stony-ironmeteorites.
These meteorites are thought to have originated from
the mantle of a rocky planet that used to occupy an
orbit between Mars and Jupiter - or they might
be from an asteroid that was large enough to have
developed a differentiated internal structure
consisting of a rock mantle and a metallic core.

Pallasites
are thought to represent the part of an asteroid or
planet that was near the mantle-core boundary where
rocky materials of the mantle were mixed with the
metallic materials of the core. Pallasites typically
have distinct crystals of olivine (usually fayalite)
surrounded by a nickel-iron matrix. A photograph of
a slice from a pallasite meteorite is
in the right column of this page.

Olivine Rain on a Developing Star

In 2011, NASA's Spitzer Space Telescope observed what is believed to have been tiny crystals of
olivine falling like rain through the dusty cloud of gas of a developing star. This "olivine rain"
was thought to have occurred as strong air currents lifted newly crystallized particles of olivine
from the surface of the forming star, high into its atmosphere, and then dropped them when the
currents lost their momentum. The result was a rain of glittering green olivine crystals.

Uses of Olivine

Olivine is a mineral that is not often used in industry. Most olivine is used in metallurgical
processes as a slag conditioner. High-magnesium olivine (forsterite) is added
to blast furnaces to remove impurities from steel and to form a slag.

Olivine has also been used as a refractory material. It is used to make refractory brick
and used as a casting sand. Both of these uses are in decline as alternative materials are
less expensive and easier to obtain.

Olivine and the Gemstone Peridot

Olivine is also the mineral of the gemstone known as "peridot." It is a yellow-green to green
gemstone that is very popular in jewelry. Peridot serves as a birthstone for the month of August.
The most valued colors are dark olive green and a bright lime green. These specimens are of the
mineral forsterite because the iron-rich fayalite is usually a brownish, less desirable color.

Much of the world's peridot used in mass-production jewelry is mined at the San Carlos Reservation in Arizona.
There, a few basalt flows containing nodules of granular olivine are the source of the peridot. Most of the stones produced
there are a few carats or less in size and often contain visible crystals of
chromite or other minerals.
They are cut in Asia and returned to the United States in commercial jewelry.

Higher quality and larger peridot crystals are mined in Pakistan and Myanmar. There, crystals of olivine
are found in metamorphic rocks. These are usually found in association with serpentine or dolomitic marble.

Lherzolite (a variety of peridotite) nodules in a xenolith collected from a basalt flow at Peridot Mesa, Arizona. These xenoliths
often contain crystals of olivine with a
color and clarity that is suitable for use
as a peridot gemstone. This specimen is
approximately 3 inches (7.6 centimeters)
across.

The gemstone known as peridot is a variety of olivine. These two faceted stones are nice specimens of yellowish green peridot. The gem on the left is a 1.83 carat cushion cut peridot of about 8 x 6 millimeters from Myanmar. The gem on the right is a 1.96 carat cushion checkerboard cut peridot of about 10 x 8 millimeters from China. Photo copyright by geology.com.

Green olivine sand from Papakolea Beach, Hawaii. The white grains are coral fragments, and the gray-black grains are pieces of basalt. If you think the grains have a "gemmy" appearance, olivine is the mineral name of a gemstone known as "peridot." The width of this view is about 10 millimeters. Photograph by Siim Sepp, used here under a Creative Commons License.

This is one of the most incredible gemstones. It is a piece of gem-quality olivine (peridot) from a pallasite meteorite, and it has been faceted into a wonderful little gemstone. This may be the most scarce gem material on Earth - but it actually originated in space. This stone is 2.85 millimeters in diameter and weighs about ten points. Photo by TheGemTrader.com.

An artist's concept of crystalline olivine rain on a developing star, inspired by the Spitzer Space Telescope. Image by NASA/JPL Caltech/University of Toledo.

Olivine from Mitchell County, North
Carolina. Specimen is approximately 3 inches
(7.6 centimeters) across.

These two specimens are peridot, a gem
variety of olivine, from a deposit in
Arizona. At this deposit the olivine occurs
in xenoliths that were erupted with a basalt flow. These
specimens are approximately 12 millimeters
across.